Different kinds of devices for both static and dynamic control of the power flow in such a transmission line are known. The object of the control may be a static distribution of power between power lines or power networks, as well as damping of power oscillations in the transmission line.
A phase shifting transformer (PST) is previously known for controlling the power flow in an ac transmission line, i.e., a three-phase ac line that interconnects two electric power networks and transmits active power between the power networks. There are a number of different previously known designs of phase-shifting transformers. FIG. 1 shows a principal circuit diagram corresponding to one such known design commonly referred to as the quadrature booster design and known from the U.S. Pat. No. 6,737,837 B1. The main parts of this device are a shunt transformer with exciter winding and regulating winding, a series transformer with a booster winding and a series winding, and an on-load tap-changer with which it is possible to regulate the phase-shift introduced by the PST. The line voltage at Node 1 is applied to the exciter winding of the shunt transformer and transformed to the regulating winding according to the turns ratio. With the on-load tap-changer it is possible to extract a portion of the regulating winding voltage and feed it into the booster winding of the series transformer. The voltage applied to the booster winding is then transformed to the series winding according to the turns ratio. The combination of Y-connected regulating winding and the delta-connected booster winding introduces a 90 degrees phase-shift of the voltage which as a result gives an induced series voltage across the series winding which is in quadrature with the voltage in Node 1. FIG. 2 describes a simplified positive sequence circuit diagram of the same quadrature booster as depicted in FIG. 1, wherein the 90 degrees phase-shift is symbolized with α=ejβ and
  β  =      ±                  π        2            .      
By controlling the magnitude of the voltage across the series winding by means of the tap-changer, the phase-shift between the voltages in Node 1 and Node 2 is controlled. By controlling the phase-shift between the voltages in Node 1 and Node 2 it is possible to control the distribution of power flow between on one hand the path in which the PST is installed and on the other hand on parallel paths in the power system network.
Advantageous is the capability of the phase shifting transformers to block parasitic power flow due to phase angle difference in a feeding network. Power may be distributed to customer in a defined way and circulating power flows may be avoided.
However, the use of a PST offers a slow control speed. The tap-changer has to go through every tap position in a sequential manner. Each tap-change is effected in the order of 3-5 seconds. Thus the PST cannot participate in a decisive way in a transient period following a power disturbance. Further frequent tap changing, in particular at high current conditions, increases the need for maintenance.
The tap-changer is a mechanical device and thus slow and an object to mechanical wear. It has a maximum regulation voltage range of 150 kV and the maximum number of operating steps is less than 35. The maximum tap voltage is in the order of 4000-5000 V between two tap positions and the maximum rated throughput current is about 3000-4500 A. The maximum power handling capacity is 6000-8000 kVA/tap and there is a short circuit thermal limit. Small voltage steps result in a greater number of mechanical operations.
Furthermore, the PST consumes reactive power due to its short-circuit reactances. FIG. 3 illustrates the control range in terms of effective phase-shift and reactive power balance for a given through current, wherein the reactive power balance of the PST is on the x-axis and the phase-shift is on the y-axis. The reactive power consumption increases quadratic with the line current and is thus pronounced at high loading of the power system.